1. Field of the Invention
This invention relates generally to the control of fluid filtering systems and more particularly to optimum control of fluid filtering systems used to filter blood, blood products or components or other biological fluids such as, for example, in a plasmapheresis system.
2. Related Applications and Prior Art
This invention is related to commonly assigned, allowed U.S. patent application Ser. No. 671,576 filed Nov. 15, 1984 (also published under International Publication No. WO 86/02858), the entire content of which is hereby incorporated by reference. In particular, although the present invention may find utility alone, it is particularly suited as an improvement in the adaptive filter concentrate flow control system and method of this related commonly assigned earlier application.
In this related prior adaptive filter control system, a transmembrane pressure (TMP) operating point is established on an empirically constructed control curve of TMP versus filtrate flow rate so as to automatically maintain substantially maximum filtrate flow rate for any given operating conditions--without permitting the TMP to exceed a threshold value at which irreversible filter clogging (and/or hemolysis) begins to occur. For a detailed description of such prior TMP control system, reference should be made to the above cited related publication. However, it will be appreciated that such prior control systems have typically attempted to maximize the efficiency with which the filter operates to perform desirable separation of filtrate from input fluid (while thereby providing a concentrate output as well). For example, in filter systems employing a rotating spinner, the filter efficiency generally increases as the spinner rpm increases and, accordingly, prior control systems have typically operated the spinner at a constant speed chosen to be substantially the highest acceptable speed (e.g., the highest speed that can be used without producing undue hemolysis or other adverse effects).
However, I have now recognized that filter efficiency (i.e., volume of filtrate removed per unit volume of input fluid) can, in some systems, actually become excessive. For example, such filter efficiency is also a function of other parameters (e.g., fluid residence time within the filter) and where some of these other parameters might vary (e.g., as the input blood flow rate and hence residence time does in a typical plasmapheresis system), it is possible that the efficiency of the filtering elements occasionally may be so high that the concentrate output from the filter becomes excessively thick. For example, in a plasmapheresis system, the hematocrit of cell concentrate may become so high that the concentrate becomes sufficiently viscous as to lose some of its fluid-like characteristics and begins to perform somewhat as a soft solid. Since cell concentrates are typically collected from a patient or other donor and periodically reinfused into that donor, this can cause problems in handling the reinfusing process (which, in turn, may cause undue hemolysis of living blood cells included in that concentrate).
In short, in a plasmapheresis system where plasma is separated from whole blood, the separation efficiency of the filter may become so high that the resulting concentrated red cells become excessively concentrated and very difficult to reinfuse into the donor through practical needle sizes in practical times and through practical blood pumps--and may become hemolyzed in the process, another detrimental effect. There may be other fluid filtering and/or processing systems where an input fluid flow rate (or other parameter) might vary so as to sometimes produce such high filtering efficiency that, for some particular applications, it would be preferable to have a lower filtration efficiency. Such problems typically may be encountered more frequently (or be of a more critical nature) where living biological fluids are involved in the filtration process.
In particular, I have now recognized that under low blood flow input rates to a plasmapheresis system, it is possible for filter efficiency to be excessive (e.g., such that the concentrate output from the filter is difficult to handle as a fluid and may become hemolyzed if forced at high flow rates through needles and blood pumps).